The present invention relates to an electrically assisted power steering system for motor vehicles. The power steering system contains an input shaft that is in working engagement with a steering wheel and that is used to transfer a torque necessary for steering wheels that are to be steered. An output member is in working engagement with the wheels to be steered. An electric motor, with which a power assist is exerted on the input shaft or the output member, is arranged on the power steering system. A sensing unit is used to sense the direction and intensity of a steering torque acting on the input shaft, and other steering-specific parameters. An electronics unit is used to process the steering-specific parameters and to control the electric motor.
An electrically assisted power steering system is described, for example, in German Published Patent Application No. 44 07 729. In this power steering system, an electric motor is controlled by an electronic control unit as a function of a torque.
Conventional hydraulic or electrohydraulic steering systems are matched to each individual vehicle version with the aid of the valve characteristic. The profile of this valve characteristic plays a substantial part, in conjunction with the chassis concept, axle kinematics, suspension kinematics, and the design of the chassis components (e.g., spring and shock absorber characteristics), in determining the steering and driving behavior of the vehicle.
An ideal valve characteristic curve, called the xe2x80x9cnominal characteristic curve,xe2x80x9d is therefore defined jointly by the vehicle manufacturer and steering system manufacturer during the development phase of each vehicle.
Valve components of conventional hydraulic and electrohydraulic steering systems cannot, however, be produced without tolerances. One is therefore forced to accept, in such systems, a production error band for the valve characteristic curve (the xe2x80x9ctolerance bandxe2x80x9d) that is usually arranged symmetrically about the xe2x80x9cnominalxe2x80x9d characteristic curve.
Vehicle manufacturers offer a wide range of different equipment variants for a vehicle model. These include, for example, different engine choices, which usually result in considerably different front axle loads. These differing front axle loads would ideally need to result in different nominal steering characteristic curves. For reasons of excessive cost and the large number of variations, however, this is usually not done.
A further influence on the profile of the nominal characteristic curve of a power steering system derives from the tire characteristic of the vehicle. Normally, however, the valve characteristic curve is coordinate only with an average tire characteristic of a current tire type.
If the original tire is replaced, for example in the context of a replacement purchase, with a tire having a different characteristic, the steering characteristics can thereby be considerably changed. The driver will then perceive the steering behavior to be unsatisfactory.
It is therefore an object of the present invention to improve an electrically assisted power steering system so that satisfactory steering characteristics are always achieved with different equipment variants of a vehicle.
The above and other beneficial objects of the present invention are achieved by providing an electrically assisted power steering systems as described and claimed herein. This object is achieved in particular in that in a power steering system, a detection unit is provided to ascertain the parameters for the equipment variant of the vehicle present in each case, and that the ascertained parameters are conveyed to the electronics unit for adapted control of the electric motor.
If the parameters for the equipment variant of the vehicle present in each case contain values for the specific front axle load of the vehicle, the steering characteristic can then be adapted to different vehicle engines.
In addition or as an alternative to the parameters for the specific front axle load of the vehicle, the parameters for the equipment variant of the vehicle present in each case can contain values for the tire characteristic of the vehicle. Different tire types and sizes can thereby be taken into account in defining the steering characteristic. The values for the tire characteristic may, in this context, contain information concerning tire slip stiffness and/or force/deformation behavior and/or parameters for specific tire types.
The parameters for the equipment variant of the vehicle present in each case may be read into the detection unit, for example, in accordance with a barcode identifier. Advantageously, the detection unit is integrated at the vehicle manufacturer""s premises into an end-of-line computer (assembly line programming unit), into service location shop testers, or, in special cases, into the electronics unit.
Multiple steering characteristics may be stored in the electronics unit. The particular characteristic that is appropriate may be activated in accordance with the parameters for the equipment variant of the vehicle present in each case.